1. Field of the Invention
The present invention relates to hydraulic systems, and more particularly to check valves which isolate an unpowered actuator from the remainder of the hydraulic system, and specifically to lock-out valves that tend to prevent such check valves from opening under high pressure conditions from the actuator.
2. Description of the Related Art
Hydraulic systems are utilized to power numerous kinds of mechanical equipment. A hydraulic actuator, such as a cylinder-piston arrangement, typically is used to move a component of the equipment. In a backhoe for example, one hydraulic cylinder moves the bucket with respect to an arm, another hydraulic cylinder provides motion between the arm and a boom, and an additional hydraulic cylinder raises and lowers the boom with respect to the vehicle frame.
A valve assembly controls the application of pressurized hydraulic fluid from a pump to the hydraulic actuator and controls the return of fluid from the actuator to a reservoir, commonly referred to as a tank. A spool-type valve often provides this control function. In this type of valve, a valve block has a bore into which a plurality of passages open leading to the cylinder chambers, the pump, and the tank. The spool is formed with several grooves and lands so that as it slides within the bore, the grooves connect the different passages. In various positions of the spool, fluid from the pump is applied to either of the two cylinder chambers and drained to tank from the other cylinder chamber. Which cylinder chamber receives the pressurized fluid determines the direction that the hydraulic actuator moves. In a centered position, the spool blocks the fluid flow to and from the hydraulic actuator. However, in the closed position some leakage occurs from the cylinder chambers to the tank passages regardless of the tolerance between the spool and the bore. Such leakage allows the machine component being driven by the hydraulic actuator to move unintentionally, which is undesirable. For example, a raised boom assembly of a backhoe may drop slowly when the control valve assembly is held for a prolonged time in the closed position.
Therefore, a common practice was to provide a conventional pilot operated, poppet check valve between the control valve assembly and the hydraulic actuator. A poppet-type valve has relatively low leakage-type as compared to a spool valve. Pressure from operation of the spool valve was applied to a pilot chamber behind of the check valve to operate a pilot piston that engaged and disengaged the poppet of the check valve to close and open a pilot passage in the poppet. Opening the pilot passage allowed the poppet to move away from the seat of the check valve. Although the combination of the pilot operated, poppet check valve with the conventional spool valve significantly reduced the leakage problem, some leakage of the pilot pressure still occurred in the spool valve.